Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Heronumber0 on 29/03/2008 12:35:40
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If metals consist of metallic bonding with positive ions surrounded by a sea of delocalised electrons, what happens when a current flows across the metal?
1 Do the electrons leave the ions that they surround and start moving round the circuit to return again? If so , is the metallic bond weakened when they do so?
2. In graphite where each layer has delocalised electrons, are free electrons lost when they travel through a current?
Basically, where do electrons go when they travel an electrical circuit? do they move at all or just pass on the energy like water carriers putting out a fire?
Why does the metallic bond hold so strong when electrons are migrating away from the ions, would the ions not repel each other.
Help please....I cannot find this information on Internet searches...
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The answer is very similar to a bucket brigade idea when thinking of the total current flow. The electrons coming out of the wire are not the same electrons going in the other end. The electrons do move though, and there is a measure of how much they move in different materials - look up "carrier mobility". They can move a lot in metals and, as you say, they can be thought of as a "sea" of electrons. However they don't move very fast or far; because there are a lot of them, even quite large currents are sustained by this large number of electrons moving quite slowly through a conductor. What charge comes out one end is being put in the other end so the total charge in the conductor is constant, but the individual electrons can take a long time to go through.
Semiconductors are more complex in that the number of carriers (which, unlike metals, can be electrons or "holes" - a lack of electrons in a continuum, and therefore behaving like a positive charge) is much smaller and their mobility is lower. I did not understand your question regarding graphite (carbon is a semiconductor) but wondered if you were referring to the physical structure impeding electron flow in one direction whilst allowing easier flow in others. I don't know the answer to that for graphite but I imagine that it could do so and produce a different ohmic resistance when current flow is across strata of graphite as opposed to along it. Certainly most crystalline semiconductors have differing carrier mobilities in different crystal planes.
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The outer 'free' electrons move from atom to atom. An individual electron doesn't move far or very fast (drift velocity) but the effect is very rapid almost like a line cars moving off slowly but the signal to move passes down the line very rapidly.
The second post appeared (with a fuller reply) before I finished mine.
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The motion of the electrons seems to depend on the type of current as well: AC vs. DC.
http://en.wikipedia.org/wiki/Speed_of_electricity (http://en.wikipedia.org/wiki/Speed_of_electricity)
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I look at metallic bonding as the ultimate in covalent bonding. The equivalent of 'valence' electrons are shared by many of the surrounding atoms, instead of just one adjacent atom as in covalent bonding. The strength of metals is due to the fact that however the material is distorted, there are always atoms (+ions) nearby for the electrons to attract - no single bonds to break; they keep hanging on.
The drift velocity when a current flows is a matter of mm per second.
With alternating current, the effect of em induction means that only electrons near the surface actually move - the so-called skin effect. In a good conductor at a high frequency, the skin may be only a few atoms in depth.
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As most people will think of alternating current as 50Hz or 60Hz it is fair to say that skin effect is fairly minimal at this frequency. It is really at much higher frequencies that this has a big effect.
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At 50Hz the skin depth is several mm, depending upon the actual metal - it's still relevant for transmission using 50mm conductors but not in your home.
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You are quite right. For 50Hz, up to 20mm diameter copper wire it makes only a few percent difference, but for 50mm wire it can add more than 50% on to the resistance. Transmission cable is not a solid copper tube, however, so the calculation is complex; really it has to be based on empirical data then. It certainly matters for power transmission.